Hello from MrBillDoesMath!
Answer:
a/2
Discussion:
The question is a bit unclear but i think they are asking for the
sum (n = 1 to infinity) of a*r^n
where "a" is the first term of the series and "r" is the common ratio. Whne f = -1 the sum is
a/(1-r) = a/(1-(-1)) = a/(2)
Thank you,
MrB